1. Field of the Invention
The invention relates to potentiometric and electrochemical reference electrodes and, in particular, to liquid junction structures such as to be used in electrochemical reference electrodes for electrochemical measurements of solutions. The invention more particularly relates to reference electrodes for use where measurement or control of potential is desired such as with pH or ISE potentiometric sensors used for laboratory analysis, for on-line process monitoring, for field measurements, or in any application where the improved precision or extended useful life of the sensor is desirable.
2. Description of the Related Art
The invention is broadly concerned with reference electrodes, such as the reference electrode portion of combination electrodes, and the reference portion of all potentiometric devices that employ a reference electrode to provide the relatively stable reference potential required in various measurements such as electroanalytical measurements, controlled potential coulometry, and polarography, and the like.
Potentiometric measurements are used widely for the determination of pH and the detection of other specific ions in a variety of settings, including chemical processes, environmental monitoring, health care and bio-processes. The accuracy of these measurements depends on the ability to measure the potential difference between a sensing electrode, whose potential varies with the analyte concentration in the measured sample solution, and a reference electrode, which ideally would maintain a constant potential. The physical interface between the reference electrode (typically the electrolyte of the reference electrode) and the sample solution is referred to as the liquid junction. The stability of the reference electrode, and consequently the accuracy of potentiometric measurements, are dependent on the constancy of the liquid junction and more particularly, the constancy of the potential across the liquid junction. However, the liquid junction and more particularly, the potential across the liquid junction are difficult to control and maintain at a constant level. Typically, it is the change in the liquid-junction potential that introduces error into the electrochemical measurement and results in the need for frequent sensor system calibration.
The errors observed in currently commercially available reference electrodes include (1) Transient or kinetic error; such error refers to relatively slow response between measurements, and slow ability to reach equilibrium, typically of five, ten, or fifteen minutes after exposure to extreme solutions. This response is primarily caused by entrapment of sample solution within the physical junction. Transient errors are typically a function of the time required to disperse this entrapped layer of sample solution and obtain a direct interface. The kinetics of this error are determined by the duration of prior immersion. The errors observed in currently commercially available reference electrodes also include (2) static error; such error typically refers to persistent offset after equilibrium is reached. Large static errors are typically caused by irreversible entrapment of sample solution deep within the physical junction structure. The errors observed in currently commercially available reference electrodes include (3) stirring error; such error refers to the shift in potential due to or associated with agitation of the sample solution. Stirring error is typically observed where there is a rate of agitation or flow of the sample. These errors exist in potentiometric electrode measurements of sample solutions, but tend to be suppressed in standard buffers where electrode accuracy is being checked Therefore, users may see no reason to disbelieve the erroneous readings obtained in non-standard solutions. See D. P. Brezinski, “Kinetic, Static, and Stirring Errors of Liquid Junction Reference Electrodes”, Analyst 108 (1983) 425-442; see also U.S. Pat. No. 4,495,052. These errors are large enough to be of practical consequence. These errors often correspond to relatively large difference in hydrogen ion (H+) concentration or activity. These errors, including those errors described above, tend to bias the measurements observed on pH meters by as much as 0.5 pH unit.
In typical, currently commercially available electroanalytical measurement systems, the interface between the reference electrode's electrolyte and the sample solution is the liquid junction. The junction potential at this sample-reference interface is related to a number of factors; it is an object of every reference electrode design to minimize the effect of the factors that would cause the liquid junction potential to drift or to vary in any way over time. Various materials have been utilized in forming a liquid junction, including porous ceramic rods, porous polymer disks, wood dowls, ground glass sleeves, capillary tubes, agar gels, asbestos fiber bundle, and other porous materials or devices, and the like. These junction structures are, in general, referred to as restriction devices because their function is to restrict the outward flow or diffusion of electroyte from the reference electrode. However, one important factor that limits the useful lifetime of a reference electrode is that junction structures typically allow the sample solution to enter the junction structure. This transport of sample solution into the junction, whether by diffusion, migration, convection or other mechanism, results in the contamination of the junction structure and a resultant undesirable variation in the liquid junction potential. Such variation typically necessitates re-calibration of the electroanalytical measurement system. If this type of contamination of the junction continues over time, the junction structure may become fouled or clogged and develop even larger offset potentials and/or potentials that chronically drift despite repeated attempts at re-calibration. In addition, sample solution will often transport past the junction structure and reach the reference half-cell itself, potentially causing additional adverse reactions.
Currently commercially available reference electrodes, especially those used for potentiometric measurements, are typically constructed based on one of two distinct designs. Each of these designs is meant to address one principle limitation encountered when using reference electrodes for making potentiometric measurements. However, each of these designs fails to address a distinct principle limitation encountered when using reference electrodes for making potentiometric measurements.
One design category is often referred to as a flowing junction reference electrode. This design provides a stream of reference electrolyte flowing through a porous junction structure or member, in an attempt to provide a relatively uniform liquid junction potential. While this design is typically effective in providing a liquid junction potential that is more uniform over time than those of the alternate design, flowing junction reference electrodes uniformly require the use of large amounts of electrolyte over relatively short periods of time. Thus, currently commercially available flowing junction reference electrodes require frequent maintenance to replenish the supply of this electrolyte solution. Furthermore, while flowing junctions are often designed to minimize this use of electrolyte by restricting the flow of electrolyte, in such flowing junctions designs the flow velocity is often reduced to a velocity that is sufficiently low enough so that the sample solution enters the liquid junction structure, typically via mass transport (diffusion, migration, or convection). The presence of this sample solution in the junction structure causes variable junction potentials, loss of calibration, clogging of the junction structure, and, over time, failure of the reference electrode. See U.S. Pat. No. 5,360,529.
The alternative design category is referred to as a non-flowing, diffusion junction reference electrode. This design depends on the substantially constant diffusion of electrolyte solution through a minimally porous junction structure to provide a steady liquid junction potential. While this design is highly susceptible to mass transport of the sample stream into the porous structure, the resulting drift in liquid junction potential may be slow enough to be tolerable in certain industrial applications. While such electrodes require frequent re-calibration, they do not require replenishment of electrolyte to the extent that flowing liquid junction electrodes do. Furthermore, such electrodes do not require systems and associated equipment to feed the reference electrolyte to the electrode, as is the case for typical liquid flowing junction electrodes.
Both reference electrode designs are in wide use but, based on their respective limitations, are typically used in different areas of application. Where precision measurements are more often needed, the flowing liquid junction reference electrode is typically used. Thus the flowing junction design is most commonly used for laboratory reference electrodes and clinical analyzers. In the laboratory environment the reference electrolyte may be relatively easily refilled as needed, even on a relatively frequent basis. Where it is desirable to minimize maintenance and where precision may be sacrificed to certain degrees, the diffusion junction reference electrode is more often utilized. Thus the diffusion junction reference electrode is typically used in industrial potentiometric sensor designs. An industrial sensor that uses a non-flowing, diffusion junction reference will typically require re-calibration on a more regular basis because of the relatively large amount of transport of the sample stream into the liquid junction structure. It is therefore not unusual for the industrial operator to install a new sensor every three months instead of attempting to re-calibrate the old sensor. For this reason, the industrial pH sensor with a built-in diffusion reference electrode is now a disposable item in most industrial applications.
In summary, two principal problems with currently commercially available reference electrodes are the frequent maintenance requirement of the flowing junction design electrodes and the frequent re-calibration requirements of the diffusion junction design electrodes. More specifically, nearly all flowing junction designs consume large amounts of electrolyte and this electrolyte needs to be replenished on a regular basis. While there are a few flowing junction designs that require small amounts of electrolyte, these designs have achieved this by reducing the electrolyte flow to the point that the level of transport of the sample solution into the liquid junction structure becomes a limitation. A slow flowing junction reference electrode performs little better than a non-flowing, diffusion junction reference electrode. On the other hand, the non-flowing, diffusion junction electrode requires no electrolyte replenishment but will be subject to slow drift errors due to transport of the sample stream into the liquid junction structure. This drift typically prevents such reference electrodes from being used for precision measurements. Frequently, such transport will cause an irreversible instability to develop in the reference electrode that will render it incapable of being re-calibrated. Because of these inherent shortcomings, sensors employing such reference electrodes are often designed to be thrown away and replaced instead of re-calibrated. As a group, all non-flowing, diffusion junction reference electrodes have a very short operational life measured in weeks and months and in the best of circumstances seldom over one to two years
Accordingly, there is a need in the art for an electrode design that exhibits both the relatively stable potential of currently commercially available flowing junction designs and the relative lack of the need to replenish reference electrolyte solution as found in currently commercially available non-flowing junction designs. Such a needed design would exhibit a relative stable junction potential over prolonged periods of time, while not exhibiting the various limitations and drawbacks of currently commercially available flowing junction and on-flowing designs.